1. Field of the Invention
The present invention relates to a rectifier circuit including a diode-connected MOS transistor with a capacitor connected between the gate and source, and a radio frequency identification (RFID) tag including the rectifier circuit.
2. Description of the Related Art
A rectifier circuit converts alternating current (AC) into direct current (DC) through the rectification of diodes. The rectifier circuit, when is provided as a semiconductor integrated circuit, employs a diode-connected MOS transistor whose source and gate are connected to each other as a rectifier diode. The diode-connected MOS transistor is, for example, an NMOS transistor isolated from a substrate through a triple well where the drain and source are connected to an N-well and the source is connected to the back gate connected to a P-well located at the bottom of the transistor. In this NMOS transistor, a diode is provided as a PN junction formed between the source and drain.
An RFID tag, which is recently watched because of its wide application, requires a rectifier circuit. The RFID tag generates a direct-current power-supply voltage for driving the integrated circuit in the RFID tag, and demodulates data signals, from an alternating current induced in a loop antenna. The rectifier circuit serves for the voltage generation and demodulation.
Such a rectifier circuit used in the RFID tag is proposed in, for example, Japanese Patent Application Laid-Open No. 2002-152080 and M. Usami et al., “Powder LSI: An ultra small RF identification chip for individual recognition applications,” ISSCC Dig. Tech. Papers, February 2003, pp. 398-399. According to the proposed rectifier circuit, if the MOS transistor is lower in threshold voltage than the PN junction, the rectification properties of the diode-connected MOS transistor depends on the properties of the MOS transistor, and accordingly is approximately the same as the rectification properties of a diode including a MOS transistor whose threshold voltage is equal to the threshold voltage of a PN junction.
However, to perform rectification of the diode, a voltage not less than the threshold voltage of the PIN junction or the threshold voltage of the MOS transistor must be applied across the PN junction, i.e. across the source and drain. The voltage to be applied across the PN junction can be supplied from, for example, a capacitor connected between the gate and source of the MOS transistor and retaining a voltage from zero to the threshold voltage (hereinafter referred as to “bias voltage”). Hence, the rectifier circuit, even if receiving an AC signal with a root-mean-square value of less than the bias voltage, can rectify such a low signal. This means that the RFID tag can receive a weak signal transmitted by a tag reader or writer and the communication range between the RFID tag and the tag reader or writer becomes wider. The wider communication range makes one reader or writer easy to detect plural RFID tags simultaneously, and widens the application range of the RFID tag.
However, since the electric charges stored in the capacitor is discharged through the leakage current of the MOS transistor to which the capacitor is connected, the voltage across the capacitor decreases with time. This means the AC signal that can be rectified increases. In other words, the conversion gain of the rectifier circuit decreases. To keep the conversion gain high, a control signal for charging the capacitor should be periodically transmitted to the rectifier circuit. If the control signal is periodically generated using a counter, charging the capacitor will be needlessly repeated. Such overcharge is undesirable because low power consumption is an important design factor of electronic devices such as the RFID tag.
The bias voltage is preferably almost the threshold voltage of the MOS transistor to be biased. This is because more than the threshold voltage causes the current in the MOS transistor to flow backward, so that the gain of the rectifier circuit decreases. However, even if the bias voltage is fixed to a preset value, the gain of the rectifier circuit may become small due to manufacturing differences of the MOS transistor.